The present invention relates to a programmable controller in which overcurrent protection of an output circuit is attained by a simple circuit configuration with general-purpose parts.
The background art will be described by using a circuit shown in FIG. 2. FIG. 2 shows an overcurrent detection circuit of an output circuit of a background-art programmable controller using a Kelvin terminal.
The reference numeral 1 represents a collector terminal of a dedicated IC having a Kelvin terminal; 2, an output terminal; 3, a Kelvin terminal for detecting an overcurrent; 4, an overcurrent detection resistor; 5, a base terminal; and 6, a load. First, when a current for operating the output circuit flows into the base terminal 5, an output current flows from the collector terminal 1 to the output terminal 2. Assume that the load 6 has turned low in resistance for some reason, for example, short circuit. Since the maximum output current is set to be up to 1 A in this output circuit, there is a fear that the output circuit is broken due to overcurrent if a current of over 1 A flows therein. There is another fear that the load 6 connected to the output terminal 2 is broken due to overcurrent. To prevent these fears from occurring, the following solution has been performed. That is, a voltage generated in the overcurrent detection terminal 3 is detected to interrupt a current flowing in the base terminal 5 before the current ends in overcurrent. The output current is thus interrupted so that the load 6 is protected from overcurrent.
In addition, in such a background-art circuit, it is not easy to identify the condition that an overcurrent is flowing in the output terminal 2. Although a method in which an ammeter is mounted in series with the load 6 to identify an overcurrent can be considered, the configuration becomes complicated and it is difficult to mount the ammeter after an overcurrent has occurred.
In addition, a background-art apparatus uses a part in which such a circuit has been integrated into a dedicated IC. Accordingly, such a part is very expensive. In addition, the maximum output current which is allowed to flow in the IC is fixed so that there causes inconvenience that a circuit having an output current larger than the maximum output current cannot be used.
A background-art programmable controller has been apt to be not only expensive but also inconvenient to use because a dedicated IC having an overcurrent detection function is used in an output circuit portion of the programmable controller.
It is an object of the present invention to provide a programmable controller which is constituted in combination of general-purpose parts so that overcurrent protection of an output circuit can be attained without increasing the cost.
According to the present invention, a programmable controller comprises a power source module, a CPU module, an input module and an output module, wherein the programmable controller further comprises an output circuit portion provided in the output module, a current detection portion for detecting a current flowing in the output circuit portion, an overcurrent control circuit portion for supplying an interruption signal to the output circuit portion when a detected current value exceeds a predetermined value, and an overcurrent indication portion for operating synchronously with the interruption signal, the output circuit portion interrupting an overcurrent at the time of an overcurrent and making the overcurrent indication portion indicate overcurrent interruption.
In addition, according to the present invention, a programmable controller comprises a power source module, a CPU module, an input module and an output module, wherein the programmable controller further comprises an output transistor provided in the output module, a current detection portion provided in a load current path of the output transistor, a comparator circuit for comparing a voltage detected by the current detection portion with a predetermined voltage, a control circuit for interrupting current conduction of the output transistor when the comparator circuit generates an output at the time of the presence of a signal instructing current conduction to the output transistor, and an overcurrent indication portion connected to the control circuit for indicating interruption of current conduction of the output transistor.